1. Field of the Invention
The present invention is used to form an efficient capacitance from a capacitor with the smallest area, in which the multi-electrode capacitor is includes a plurality of metal laminates coupled to each other and the characteristics of the edge-coupled effect therein is employed.
2. Description of Related Art
In modern high-frequency and high-speed printed circuit board (PCB) design with some specific functions like those including an IC substrate or a PC motherboard or some other communication systems, some capacitive elements will be added between the power end and the ground end for the purpose of filtering varying electric noise from the inner system or stabilizing electric voltage. The capacitive elements mentioned above are called a decoupling capacitor.
There are two main technologies of the capacitive elements used for decoupling, namely use of a discrete component capacitor, like the surface mount technology (SMT) capacitor, and use of a capacitive substrate developed recently for decoupling.
The methods for the discrete component and the capacitive substrate of the prior art described above have their own merits and demerits. Reference is made to the following description: the three main existing demerits of the discrete component module are (1) the working bandwidth is narrow; (2) the alternating resistance is not low enough; (3) the module cannot be integrated with the system in package. On the other hand, the three main demerits of the current capacitive substrate module are (1) the via parasitic effect is too big; (2) the module only has the design with two-dimension structure and the area is too big; (3) the electric loss of the module is large. Nevertheless, the PCB with capacitive substrate including the built-in components can be used for reducing the use of the passive elements as well as reducing cost, reducing the product volume, reducing the use of welding points, raising the reliability of the product and further promoting the electric characteristics of the passive elements.
The PCB employed in prior art often uses planar substrates with the same dielectric coefficient, such as glass reinforced epoxy (FR4) substrates, which have restrain noises poorly at high frequencies and have low integration of passive elements. U.S. Pat. No. 5,079,069, which is illustrated in FIG. 1, relates to a capacitor laminate for use in capacitive printed circuit boards and a method for manufacturing the same. Further improvement of the capacitive substrate is disclosed. A plurality of electrical components 12 is installed on a PCB 10, and both sides of the PCB 10 can be mounted with the active elements such as, for example, ICs, transistors or passive elements like capacitors and resistors. Usually, the PCB 10 is combined with one or a plurality of laminates including power layers, ground layers or other conductive material, and all kinds of elements thereon are connected with each other, or to the power source or ground.
The following description is pf the built-in capacitive substrate in substitution for previous capacitive elements on the PCB. The current PCB includes function other than conventional electric signal transmission, so many passive or active components are installed therein. Reference is made to the cross-sectional view of the PCB made of the built-in components shown in FIG. 2. The surface-mounted devices (SMD) 20 are mounted on the surface of PCB 10, in which the power wires 34 in the vertical direction and the ground wires 36 in the horizontal direction connect with the built-in capacitor 26 made of the first plate 28 and second plate 30 by way of these vias. The first plate 28 and the second plate 30 constitute the power end and the ground end, respectively (shown by the black spots in drawing), i.e. the plates 28, 30 are separated by a space at an interval and constitute the built-in capacitor 26 coupled with the power/ground ends and a dielectric 32 in the PCB 10. A signal wire 38 passes through the PCB 10 to be the wiring or signaling among the electrical components.
FIG. 3 shows the cross-sectional view of the multi-layer built-in capacitor in PCB corresponding to the drawing in FIG. 2. Separated, coupled, built-in capacitors 26 and 33 produce twice the former capacitance.
U.S. Pat. No. 5,161,086 discloses a structure used to gain higher capacitance through the specific surface treatment in the built-in capacitor shown-in FIG. 4. A built-in capacitor 41 is formed by two plates combined with a dielectric 42 within PCB 40. The surface treatment of the inner layers 43, 44 of the built-in capacitor 41 is used to change the characteristics of the plates for obtaining higher capacitance.
For the application of high-frequency analogy or digital hybrid electrical system, the lateral part of the multi-layer plates occupied within the PCB can be used to gain more capacitance.
The present invention discloses a structure and method for manufacture of multi-electrode capacitor and employs the edge-coupled effect from a plurality of metal laminates to provide efficient capacitance in the limited area within the PCB. Furthermore, the implementation of invention restrains the interference of noise at high frequencies and reduces the number of the wiring layers in a PCB, and promotes integration and reduces the volume of the system.